A family of ornomultimeric outa-membrane proteins, "secretins", mediates the secretion of large macromolecules such as enzymes and filamentous bacteriophages across bacterial outer membranes to the extracellular milieu. The pIV protein is coded for by the filamentous. virus fibut is not part of the structure of the virus. It is known to exist as a hornornultimeric structure. It was examined in the STEM both as a fi-eeze-dried sample, to determine its mass, and as a stained sample to observe details of its structure. It exists as a unit multimer (-14 subunits) and a multimer dimer. It is roughly cylindrical with a pore of -80 A, sufficient to accornodate filamentous phage. See Highlights... Work is underway to locate portions of the monomer in the complex by Au-labeling cysteines that have been genetically engineered in different locations.